1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus that includes the optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus is used in laser printers. A typical optical scanning apparatus is configured to deflect a light beam output by a light source with an optical deflector, form an optical spot on a scan target surface by focusing the deflected light beam on the surface using a scanning image-forming optical system, such as an f-theta lens, and scan the scan target surface with the optical spot. The scan target surface is usually a photosensitive surface of a photosensitive medium, such as a photoconductor.
In a typical full-color image forming apparatus, four photoconductors are arranged in the direction of feeding a recording sheet, and a deflecting unit is provided that deflects a flux of light beams emitted by a plurality of light sources corresponding to each of the photoconductors. A plurality of scanning image-forming optical systems corresponding to each of the photoconductors exposes the photoconductors at the same time to form latent images corresponding to single colors. The single-color latent images are then made visible by a developing unit using developers of different colors such as yellow, magenta, cyan, and black. The single-color visible images are then transferred onto a single recording sheet and fixed thereby obtaining a full-color image. Such an image forming apparatus that forms a two-color image, a multicolor image, or a color image using at least two sets of the optical scanning apparatus and the photoconductors is known as a tandem image forming apparatus.
Some tandem image forming apparatuses include a single optical deflector shared by a plurality of the photosensitive media. A so-called opposing scanning method of inputting a flux of light beams from opposite sides of the optical deflector and spreading the flux for scanning is disclosed in Japanese Patent Application Laid-open No. H11-157128 and Japanese Patent Application Laid-open No. H9-127443. Another method of inputting a plurality of virtually parallel fluxes apart from one another in the sub scanning direction and arranging a plurality of scanning optical systems corresponding to the fluxes in the sub scanning direction for scanning is disclosed in Japanese Patent Application Laid-open No. H9-54263. Still another method of inputting the flux from one side of the optical deflector and scanning with a set of three scanning optical systems is disclosed in Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, and Japanese Patent Application Laid-open No. 2001-33720. In this method, a plurality of fluxes that fall on different scan target surfaces pass through two of the scanning optical lenses, and a remaining lens is provided with respect to each scan target surface.
If the optical deflector is shared by a plurality of the scan target surfaces, the number of the optical deflector is reduced, and therefore the size and the production cost of the image forming apparatus can be also reduced. In these years, the size reduction of the image forming apparatus has been accelerated, and reduction of the size and the production cost of the optical scanning devices used in the image forming apparatus is demanded.
To save the production cost and footprint, it is desired to use a photodetector that detects synchronization signals at one of a start point of drawing and an end point of drawing (hereinafter, “one-point synchronization”). With the one-point synchronization, not only the production cost of the synchronization detecting units can be reduced, but also the electrical control system can be simplified.
However, to provide the synchronization detecting units in addition to the scanning optical systems in the opposing scanning method, if they are arranged symmetrically, a light on one side is disadvantageously apt to interfere with a light source board 32 that holds a light source 33 as indicated by a dotted circle shown in FIG. 7.
To avoid interference, the light source 33 can be arranged away from the light source board 32; however, the size of the optical scanning apparatus is made larger. Alternatively, many reflecting mirrors 39 can be used to avoid the interference. However, it is difficult to spare a space for the reflecting mirrors 39, and a synchronization beam can depart from a photodetector due to accumulation of tilts of the reflecting mirrors 39. Furthermore, increase of the optical systems causes degradation of the optical performance and cost increase. In this manner, use of many reflecting mirrors 39 is disadvantageous for reduction of the size and the production cost, and degrades detecting accuracy at the same time.
In FIG. 7, reference numeral 34 denotes a coupling lens, 35 denotes a cylindrical lens, 38 denotes a synchronization lens, and 40 denotes a photodetector.
It is also possible to reflect the light by the reflecting mirror 39 so that the synchronization detecting units are arranged away from a scanning lens 36. However, in fact, a reflecting mirror that reflects the light in the sub scanning direction to conduct the light onto a scan target surface 7, i.e. the photoconductor, is arranged at a close position opposite of a polygon mirror (i.e., an optical deflector) 31 of the scanning lens 36, and therefore the reflecting mirror is apt to interfere with the synchronization detecting unit.
Another way is providing the synchronization detecting units without using the reflecting mirrors as shown in FIG. 8. However, the focal length of the synchronization detecting unit in the sub scanning direction is remarkably shorter than that of the scanning optical system, which amplifies an error of synchronization detection at the time of drawing, resulting in degradation of accuracy of the synchronization detection.
If the synchronous optical path is made longer without using the reflecting mirrors, the optical scanning apparatus is made larger. To extend the synchronous optical path and reduce the size of the optical scanning apparatus at the same time, there is a need of bending the optical path in the direction orthogonal to the optical axis of the scanning optical system using the reflecting mirror, which causes the same problem as described above.
To meet the demand for the reduction of the size and the production cost of the optical scanning apparatus, one of possible approaches is to configure the scanning optical systems with one lens instead of two lenses, which is common. If the optical scanning apparatus includes a single smaller lens arranged as close to the optical deflector as possible, the footprint and the production cost will be greatly reduced. On the other hand, though it is less effective to reduce the size and the production cost, even if two scanning lenses are used to obtain desired optical performance, an equivalent effect can be obtained by arranging one of the scanning lenses as close to the optical deflector as possible. However, to arrange the scanning lens closer to the optical deflector, the light beam for synchronization needs to pass through the scanning lens, and there is a risk of a large misalignment of a scanning point in the main scanning direction accompanying the change of temperature.
An example of the scanning optical system including a single scanning lens based on the one-point synchronization is disclosed in Japanese Patent Application Laid-open No. H11-44857. However, the light beam directed to synchronization passes through the scanning lens, and therefore, in the case of temperature change, the scanning point is misaligned in the main scanning direction at the point of the synchronization due to the deformation of the scanning lens. The misalignment is not remarkable in monochrome scanning as in Japanese Patent Application Laid-open No. H11-44857. However, the single scanning lens based on the one-point synchronization cannot be employed in color scanning, especially in the opposing scanning method, because the scanning directions of opposing scanning optical systems are opposite in the main scanning direction, the misalignment appears a color shift, which drastically lowers the image quality.
A known example of the scanning optical systems that employs the opposing scanning method based on the one-point synchronization is disclosed in Japanese Patent Application Laid-open No. H11-44857. However, the light beam directed to the synchronization detecting unit passes through the scanning lens, and therefore, in the case of temperature change, the scanning point is misaligned in the main scanning direction at the point of the synchronization due to the deformation of the scanning lens. Furthermore, because the synchronization beam is reflected after passing through the two scanning lenses, it is hard to configure the optical scanning apparatus when a reflecting mirror in the sub scanning direction is provided in an actual drawing apparatus. There will be a need of either deflecting the light beam also in the sub scanning direction with more reflecting mirrors for the synchronization beam or increasing the size of the optical scanning apparatus to prevent interference with other optical systems. As described above, therefore, the opposing scanning method based on the one-point synchronization is disadvantageous for reduction of the size and the production cost, and degrades detecting accuracy.